Obama plans to announce Johnson's nomination Friday. He must be confirmed by the Senate before taking over the post most recently held by Janet Napolitano, who stepped down in August to become president of the University of California system.

As general counsel at the Defense Department during the wars in Iraq and Afghanistan, Johnson oversaw the escalation of the use of unmanned drone strikes, the revamping of military commissions to try terrorism suspects rather than using civilian courts and the repeal of the military's ban on openly gay service members.

A senior Obama administration official on Thursday confirmed Johnson's selection, first reported by The Daily Beast. The official was not authorized to speak about the nomination on the record and spoke on condition of anonymity.

The official said Obama chose Johnson because of his experience as a national security leader. The official noted that Johnson oversaw the work of more than 10,000 lawyers and was responsible for reviewing every military operation approved by the president and defense secretary.

He is not as well versed in the disaster response side of the Homeland Security Department, although the administration official pointed out the Pentagon also coordinates federal relief to respond quickly to disasters.

At the Pentagon, Johnson was involved in the decision to move Army Pfc. Chelsea Manning from a military jail in Quantico, Va. to Fort Leavenworth, Kan. Supporters of Manning, the young soldier who leaked thousands of classified documents to WikiLeaks, said conditions at the military jail in Virginia were too harsh.

Johnson also warned former Navy SEAL Matt Bissonnette that by publishing a book on the SEAL's raid that killed Osama bin Laden he was in "material breach" of two nondisclosure agreements he signed earlier in his career.

Johnson also helped pave the way for the military's 2010 repeal of the "don't ask, don't tell" policy banning openly gay service members.

"For those service members who are gay and lesbian, we lifted a real and personal burden from their shoulders," Johnson said last year at an event recognizing the service of gay troops. "They no longer have to live a lie in the military."

Johnson, a former assistant U.S. attorney who also was Air Force general counsel under President Bill Clinton, left the Obama administration in 2012 and returned to private practice.